История изменений

import Control.Monad.Trans.Either
import "mtl" Control.Monad.State
import Data.DList (DList)
import qualified Data.DList as DL

data Q = Q Int deriving Show

type M a b = EitherT a (State (DList Q)) b

l :: Q -> M a ()
l y = lift $ modify (\x -> x `DL.snoc` y)

e :: a -> M a ()
e x = left x

test1 = do
  l (Q 1)
  l (Q 2)
  e ()
  l (Q 3)

λ> fst $ runState (runEitherT test1) DL.empty
Left ()
λ> DL.toList . snd $ runState (runEitherT test1) DL.empty
[Q 1,Q 2]

import Control.Monad.Trans.Either
import "mtl" Control.Monad.State
import Data.DList (DList)
import qualified Data.DList as DL

data Q = Q Int deriving Show

type M a b = EitherT a (State (DList Q)) b

l :: Q -> M a ()
l y = lift $ modify (\x -> x `DL.snoc` y)

e :: a -> M a ()
e x = left x

test1 = do
  l (Q 1)
  l (Q 2)
  e ()
  l (Q 3)

λ> fst $ runState (runEitherT test1) DL.empty
λ> DL.toList . snd $ runState (runEitherT test1) DL.empty
[Q 1,Q 2]